My understanding, to the small amount that I have researched this, is that the FOBT picks up blood presumably from bleeding polyps. Why are the polyps bleeding to begin with? Are they somehow more sensitive to the waste accumulation/passage, and are easily irritated. Inquiring minds wanna know.
The Fecal Occult Blood Test picks up small amounts of blood from any cause, benign or malignant.
Polyps can be precursors to cancer, but their mucosal surface is not typically friable enough to bleed when they are still ordinary polyps. Some polyps can progress to become malignant. Once they become malignant, of course–or if there is malignancy involving the normal mucosa, blood can appear in the stool because the normal surface is disrupted. There are many causes for blood in the stool, though, besides malignancy, and a stool screen for occult blood is not a particularly specific test.
Occult blood can be an early sign of malignancy, but it could be a late sign as well. Therefore in many individuals routine screening for colon cancer with colonoscopy or some other visualization is recommended. A FOBT is only one way to screen for colon malignancies. It’s not a screen for polyps per se, although any abnormal structure sticking out into the gut lumen is a little more likely to bleed than completely normal tissue.
Think about it - if you’ve got a polyp waving out there in the lumen of the colon, getting whacked by Mexican food remnants, spare change or whatever is passing through, it’s reasonable to expect there might be some bleeding from time to time. This is more to be expected from a large polyp on a stalk, as opposed to a small sessile (flat) polyp. If a person has significant bleeding and endoscopy finds only a small sessile polyp or two, that’s not a very likely explanation for the bleed.
As CP noted, there are a lot of potential explanations for a gastrointestinal bleed, benign and malignant.